FM Global DS 2-1: Corrosion in Sprinkler Systems — Oxygen, MIC and Nitrogen

A twelve-year-old shopping centre begins leaking from four separate points on its wet pipe sprinkler system during an annual test. Camera inspection of the branch lines shows tubercles and irregular internal wastage, a picture that at first suggests microbiologically influenced corrosion (MIC). FM Global DS 2-1 (Corrosion in Automatic Sprinkler Systems, October 2016, Interim Revision July 2022) adds a caution: pitting under tubercles can have other causes, and only a metallurgical examination establishes the mechanism (3.8.4 and 2.2.1.6). This article summarises the DS 2-1 recommendations by system type.

Where Corrosion Comes From: Water and Oxygen (2.1 and Section 3)

• Basic principle (2.1): most sprinkler system corrosion traces back to water and air in the system. In a wet system, oxygen in the water and trapped air is consumed fairly quickly and corrosion stops until fresh air or water is introduced. In a dry system supervised with compressed air, water left in the piping causes rapid corrosion.
• Types of corrosion (Section 3.0): uniform, galvanic, pitting, crevice, selective leaching, erosion, environmentally induced cracking, intergranular and MIC.
• Trapped air in wet systems (3.4): DS 2-1 shows leaks beneath tubercles under an air pocket filling about 40% of the pipe volume. Remedy: FM Approved automatic air-release valves of at least 1/2 in. (13 mm), or manual valves, at high points, venting air every time the system is drained and refilled (2.2.1.9).
• Weld seam corrosion (3.7): grooving corrosion along electric-resistance-welded (ERW) seams is common, especially with the seam near the bottom (6 o’clock). In dry systems the seam is oriented toward the roof, at least 45° from the floor (2.2.1.12.2 H).
• MIC (3.8): corrosion initiated or accelerated by microorganisms in a biofilm on the metal surface. In more than 300 cases FM has examined since 2001, MIC accounts for 10% to 20% of corrosion damage. Sulfate-reducing (SRB) and acid-producing (APB) bacteria are most commonly implicated; stagnant water and dead ends favour microbial activity.

Recommendations by System Type (2.2.1)

System	Pipe	Additional measures
Wet	No internally galvanized steel pipe (2.2.1.4)	Air-release valves at high points (2.2.1.9); FM Approved pressure-relief valve of at least 1/4 in. (6.4 mm) for thermal expansion, set above the maximum rated working pressure (175 psi [12 bar] is typical) (2.2.1.10)
Wet with nitrogen	No internally galvanized pipe	FM Approved nitrogen generator, air-release valves, venting at every drain and refill (2.2.1.13); nitrogen concentration at least 98% (3.3)
Dry / preaction with nitrogen	Black steel if nitrogen is used for the life of the system; otherwise galvanized or polymer enhanced steel	Pressurised with an FM Approved nitrogen generator (2.2.1.12.1)
Dry / preaction without nitrogen, and deluge	Galvanized or polymer enhanced steel; black steel with an FM Approved vacuum system	Pitch per DS 2-0; low-point drains, drained regularly; avoid rolled groove joints; in humid regions an air dryer bringing the supply-air dew point 20°F below the lowest expected room temperature; air leaks fixed; piping kept dry all year, with no alternating between wet and dry (2.2.1.12.2)

For all systems: new, clean pipe and components with compatible metals; where contamination is suspected, disinfect with isopropyl alcohol or equivalent, never chlorine (2.2.1.1); cap pipe ends if stored outdoors (2.2.1.2); Schedule 40 or equivalent over leak-sensitive occupancies (2.2.1.3); avoid untreated water sources and do not use chemical cleaners or corrosion inhibitors (2.2.1.11).

Nitrogen: FM Test Results (3.2–3.3)

• In FM testing, carbon steel in a dry system using air corroded 20 times faster than with nitrogen (3.2).
• In a wet system, carbon steel with air corroded up to 14 times faster than in a system purged with nitrogen (3.3).
• The recommended nitrogen concentration is at least 98%; testing at 95% gave corrosion rates close to those with compressed air (3.3).
• Manual method for wet systems: drain completely, inject nitrogen to about 30 psi (2 bar), read the concentration at a remote vent, purge to zero and repeat until at least 98% (typically 3–4 cycles), then refill with water. With an auto-purge device the process runs with the system in service over about 14 days (3.3.1).

MIC and Chemical Treatment (3.8, 3.11)

• Finding MIC-related bacteria in the water or corrosion products does not by itself confirm MIC; chemical, microbiological, metallurgical and operational data are evaluated together (3.8.4).
• In dry and preaction systems with galvanized pipe, residual water and dissolved oxygen are the leading causes of corrosion even where bacteria are present in the water supply (3.8.4).
• Chemical treatment for MIC is not recommended, as it can accelerate corrosion if done improperly; replace the pipe instead (3.8.5). Cleaners and inhibitors can collect in dead ends and pendent drops, causing sprinkler corrosion or restricted orifices (3.11).

When Corrosion Is Found (2.2.1.5–2.2.1.8)

• Systems with pinhole leaks or signs of corrosion such as scale or tubercles are promptly checked for obstruction per DS 2-81: a sufficient sample inspected with a video borescope; full flushing if obstructions are found; replacement of sections whose obstructions do not flush out (2.2.1.5).
• A metallurgical examination of a sample identifies the type and extent of the corrosion mechanism (2.2.1.6).
• Annual visual inspection for obstruction until the corrosion is under control (2.2.1.7).
• Table 1 (p. 4): replace any section where the wall remaining in any single pit is below 25% for Schedule 40, 50% for Schedule 10, or 75% for Schedule 5 and hybrid schedule pipe (2.2.1.8).
• Routine checks are in DS 2-81: wet systems fed by an open water supply every 5 years (Table 2.5.2.1); dry and preaction systems with black steel pipe (excluding systems originally installed with nitrogen and refrigerated-area systems) at 10 and 20 years and every 5 years thereafter (Table 2.5.3.3).

Underground Piping (2.2.2)

• No iron or steel pipe under coal piles, in cinder fill or where acids and alkalis can reach the soil; FM Approved nonmetallic underground pipe where the water table is high or the soil corrosive.
• After a corrosion leak, uncover representative sections; replace or reline significantly corroded pipe per DS 3-10 using FM Approved relining systems.

A Costly and Recurring Mistake

A recurring pattern on dry pipe systems installed a decade or more ago: several leaks appear in quick succession, and camera inspection shows widespread oxygen corrosion and pitting along the main. The remedy is renewal of the main and several branches, at many times the cost of prevention. Looking back, the same omissions appear: the system was supervised with air rather than nitrogen, and low-point drains were missing. Applying the measures of DS 2-1 2.2.1.12 (nitrogen, or galvanized or polymer enhanced pipe, with proper pitch, low-point drains and dried air) at design stage would have included them from the start.

Quick Checklist

• No internally galvanized pipe on wet systems; air-release valves at high points
• Dry and preaction systems use either nitrogen for life or galvanized/polymer enhanced pipe
• Dry systems without nitrogen have pitch, low-point drains and an air dryer
• No chemical cleaners, inhibitors or biocides
• Where corrosion is found: borescope, flushing and metallurgical examination
• Pipe replacement decided against the Table 1 remaining-wall criteria
• DS 2-81 obstruction checks scheduled

Frequently Asked Questions

What is MIC and how is it identified?

MIC is corrosion initiated or accelerated by microorganisms in a biofilm on the metal surface, typically producing pitting and pinhole leaks beneath tubercles. Under FM DS 2-1 3.8.4, finding bacteria in the water or corrosion products does not confirm MIC on its own; chemical, microbiological, metallurgical and operational data are evaluated together. In the cases FM has examined, MIC accounts for 10% to 20% of corrosion damage.

What drives corrosion in a wet system?

Under DS 2-1 3.3, the main causes are trapped air, corrosive water chemistry and oxygenated water introduced during maintenance. Section 2.2.1.9 calls for FM Approved air-release valves of at least 1/2 in. (13 mm) at high points, venting every time the system is drained and refilled; nitrogen purging to at least 98% is an option (2.2.1.13).

Is nitrogen mandatory on a dry system?

No, but the choice drives the pipe selection. Under DS 2-1 2.2.1.12.1, black steel is acceptable if an FM Approved nitrogen generator supplies nitrogen for the life of the system; without nitrogen, use galvanized or polymer enhanced steel with proper pitch, low-point drains and, in humid regions, an air dryer (2.2.1.12.2). In FM testing, carbon steel in a dry system on air corroded 20 times faster than on nitrogen.

Is galvanized pipe suitable for sprinkler systems?

It depends on the system. DS 2-1 2.2.1.4 excludes internally galvanized pipe from wet systems, while for dry, preaction and deluge systems without nitrogen it is the recommended pipe together with polymer enhanced steel (2.2.1.12.2). Galvanized pipe must be kept dry: Section 3.6 notes that new dry systems with residual water can develop pinhole leaks within 2 to 3 years.

Can biocides or corrosion inhibitors be used?

No. DS 2-1 2.2.1.11 says not to use chemical cleaners or corrosion inhibitors, and 3.8.5 does not recommend chemical treatment for MIC, recommending pipe replacement instead. Section 3.11 explains that such chemicals collect in dead ends and pendent drops, causing sprinkler corrosion or restricted orifices.

SprinkCalc — Fire Sprinkler Design Across Three Standards

SprinkCalc covers hazard classification, design density and area, K-factor selection, water demand and hydraulic calculations for NFPA 13, FM Global and BS EN 12845 in a single iOS app, and exports a professional PDF report.

Download SprinkCalc on the App Store

MEP Calc — 110+ Engineering Calculators

MEP Calc bundles 110+ engineering modules in one iOS app: 21 fire calculations plus heating, cooling, HVAC, plumbing, steam and natural gas.

Download MEP Calc on the App Store